In growth of a silicon single crystal by a Czochralski process, a quartz crucible is required to receive a silicon melt melted by a heater. However, the quartz crucible reacts with the silicon melt and is dissolved in the melt. Consequently, oxygen is eluted from the crucible and inflowed into a single crystal through a solid-liquid interface. The oxygen inflowed into the single crystal increases the strength of a wafer and creates a bulk micro defect (BMD) in the wafer, which acts as a gettering site of metal impurities during a semiconductor process and causes various crystal defects and segregation, thereby reducing yield of semiconductor devices. Therefore, during growth of a silicon single crystal by a Czochralski process, concentration of oxygen inflowed into a single crystal through a solid-liquid interface should be appropriately controlled.
Conventionally, a crucible rotation rate, an amount of argon (Ar) gas inflow or pressure was controlled for oxygen concentration control. And, conditions of a cusp magnetic field were changed to vary oxygen concentration while little degrading the crystal defect level. Further, it has been reported that oxygen concentration is influenced by a crucible rotation rate under the conditions of a strong horizontal magnetic field by use of a superconductive horizontal magnet.
As a technique related to oxygen concentration control, Japanese Laid-open Patent Publication No. 9-235192 discloses control of rotation rate of a crucible and a single crystal to a predetermined range when pulling up a silicon single crystal by MCZ (Magnetic Field Applied Czochralski Method) so as to reduce oxygen concentration and oxygen concentration deviation in a radial direction. According to this technique, for a small-diameter silicon single crystal of about 6 inch diameter, level of oxygen concentration deviation in a radial direction can reach up to ±0.5 ppma. However, for a large-diameter silicon single crystal of about 12 inch diameter, level of oxygen concentration deviation in a radial direction may be deteriorated.
As another example, Korean Patent No. 735902 teaches a technique that controls a crucible rotation rate and additionally controls an amount of argon (Ar) gas inflow and pressure so as to effectively control the oxygen concentration according to length of a single crystal under the conditions of a strong horizontal magnetic field. However, this technique is suitable to control the oxygen concentration of a small-diameter silicon single crystal of about 8 inch diameter.
The conventional technique for controlling the oxygen concentration by application of a strong horizontal magnet field mainly intends to grow a single crystal of 8 inch diameter or less using a crucible of 24 inch diameter or less containing a small volume of silicon melt. If the above-mentioned technique is used to grow a single crystal of 12 inch diameter or more using a crucible of 32 inch diameter, oxygen concentration control may fail. This is because an increase of 80% or more in volume of a silicon melt results in an unsteady flow of the melt. That is, as volume of a melt is larger, flow of the melt is unsteadier, and consequently oxygen behavior becomes complicated. Thus, simply changing an amount of argon gas inflow or pressure according to length of a single crystal (volume of a melt) does not lead to a proper control of oxygen concentration. Meanwhile, disorder occurs to oxygen behavior under the conditions of a strong horizontal magnetic field. A scheme should fundamentally solve the disorder problem and improve the width of change in pulling speed caused by a severely unsteady flow of a melt.
As length of an ingot is larger, a contact area between a melt and a crucible reduces. To overcome the reduction effect, the conventional technique increased gradually a crucible rotation rate (set a crucible rotation rate in the range between 0.1 rpm and 0.9 rpm or between 0.3 rpm and 0.7 rpm). In this case, as shown in FIG. 1, oxygen concentration reduces according to probability at an intermediate stage of a body of an ingot. In FIG. 1, a section between upper and lower horizontal reference lines (as indicated by one-dotted chain lines) means a preferable oxygen concentration range. Application of the above-mentioned crucible rotation rate results in oxygen concentration of 11 ppma or more for a single crystal of 8 inch diameter or less, however it results in a very unsteady oxygen concentration profile for a single crystal of 12 inch diameter or more because the large-diameter single crystal uses a large volume of melt.
And, in the case that an amount of argon gas inflow and pressure are controlled according to the above-mentioned technique so as to control the oxygen concentration (an amount of argon gas inflow is decreased from 160 lpm to 140 lpm, and pressure is increased from 50 Torr to 60˜70 Torr), oxygen concentration reduces at an intermediate stage of a body of an ingot as shown in FIG. 2.
Analysis tells that the above-mentioned phenomenon is resulted from split of a melt into a low oxygen melt and a high oxygen melt under the conditions of a strong horizontal magnetic field. That is, according to a Czochralski process not using a magnetic field or a cusp or vertical MCZ having a rotational symmetry, rotational symmetry can be maintained by rotation of a single crystal and a crucible, and thus a melt is not split into two type melts. However, according to a horizontal MCZ having a mirror symmetry, Lorentz force is generated in the opposite (right and left) direction due to rotation of a single crystal and a crucible, and thus a melt is split into two types of melts. On this condition, the behavior of oxygen inflowed into a single crystal is influenced according to how a low oxygen melt and a high oxygen melt govern a lower portion of an interface of the single crystal. For example, for a 3rd Run of FIG. 1, a high oxygen melt is dominant in ingot length between 800 mm and 900 mm and a low oxygen melt is dominant in ingot length between 1000 mm and 1300 mm. It was found that the type of a dominant melt is influenced by a crucible rotation rate. The frequency of a low oxygen melt is 0.005 Hz, which corresponds to a crucible rotation rate of about 0.3 rpm. Therefore, when a crucible rotation rate is about 0.3 rpm, a resonance phenomenon with a low oxygen melt occurs, which makes it difficult to grow a crystal of high oxygen.